This invention relates generally to an optical recording medium and more particularly to an optical recording medium having a reduced thickness and an increased hardness in order to improve the information storage density thereof, and to defend against damage resulting from contact with an optical lens or the like. Most recently, optical recording media have become very important in the recording medium industry, and therefore various studies and much research concerning optical information recording systems for recording information on, and retrieving information from, optical recording media have taken place. An optical information recording system has various advantages, including information being recorded and reproduced without contact between the optical reading/recording head and the recording medium, a higher recording density being achieved as compared with magnetic recording medium, and the optical information recording system being applied to a system allowing for read only optical recording medium, a write once read many (WORM) optical recording medium, or a rewritable optical recording medium. Thus, an optical information recording system utilizing optical recording media having a large capacity allows for the recording and storage of a large amount of information at an inexpensive cost, and is therefore useful in various fields including various industrial applications and commercial uses.
Read only memory type optical recording media are quite common, and include digital audio compact discs (CDs), optical video discs, DVDs, CD-ROM computer discs and the like. A conventional optical disc 300 such as a CD, optical video disc, DVD, CD-ROM or the like is shown in FIG. 3, and is formed with one main surface of a disc substrate 310 including concave and convex patterns 315 which comprise pits and grooves, or the like, for recording information. This pattern of pits and grooves represents an information signal. A reflective film 311 comprising a metal thin film such as an aluminum film or the like is formed as a recording layer on the main surface of disc substrate 310 having the patterns formed therein. Further, a protective film 312 for protecting the reflective metal film from water and oxygen in the air is formed adjacent the reflective metal film. This protective film is usually formed of a polycarbonate or U.V. curable resin. Information is retrieved from optical disc 300 by irradiation of a reading light beam 316 which is focused through an objective lens L. The light is reflected from recording film 311 in accordance with the information recorded as pits and grooves.
Light transmissive layer 312 is formed of a transparent material, and recording film 311 may be formed of a plurality of layers, including a transparent dielectric film of silicone nitride or the like positioned at the lower surface of light transmissive layer 312, a magneto-optical recording film formed of TbFeCo or the like formed as an information recording layer positioned adjacent the first layer of silicone nitride, and a second transparent dielectric film formed of silicone nitride positioned adjacent the film of TbFeCo, to form a type of layer sandwich. Reflective film 311 is positioned between the second transparent dielectric film of recording film 311 and recording medium substrate 310.
A rewritable magneto-optical disc may be similarly formed with the same general structure, and includes a recording layer formed of a transparent dielectric film, comprising silicone nitride or the like, a magneto-optical recording film, comprising TbFeCo or the like positioned adjacent the transparent dielectric film, and a further transparent dielectric film, comprising silicone nitride or the like, formed adjacent the magneto-optical recording film, thus sandwiching the magneto-optical recording film between two layers of transparent dielectric film. A reflective film, comprising an aluminum film or the like, is then formed between the recording layer and the disc substrate.
In an attempt to increase the recording density of an optical disc, magneto-optical disc or the like, it has been proposed that the numerical aperture of the objective lens used for reading information from or writing information to the disc be increased. This proposed increase in the numerical aperture of the objective lens would reduce the diameter of the spot reproduction light, and therefore recording and reproduction using this reduced diameter spot recording light would take place with a higher recording density. However, if the numerical aperture of the objective lens is increased, it is consequently necessary to decrease the thickness of the light transmissive layer (or protective layer) through which the irradiated reproduction spot light is transmitted. This is because as the numerical aperture of the optical pickup increases, the allowable skew angle at which the disc surface may be displaced relative to the coaxial axis of the optical pickup is reduced. This skew angle is proportional to the square of the product of an inverse number of the wavelength of light from a light source, and the numerical aperture of the objective lens. Accordingly, in order to guarantee the required reduction of the skew angle, the thickness of the light transmitting layer must be reduced.
In order to achieve this reduced thickness of the light transmissive layer, there has been proposed in the prior art an optical recording medium having a recording layer formed of concave and convex pits or grooves on one main surface of a disc substrate, a reflective layer provided adjacent the recording layer, and a light transmissive layer comprising a thin film formed adjacent the reflective layer. A reproduction light is irradiated on the disc, and is transmitted through the light transmissive layer to reproduce information recorded as pits and grooves on the recording layer. The formation of the optical medium in this manner allows for a decrease in the thickness of the light transmissive layer, and also enables an increase in the numerical aperture of the objective lens, as described above. Most commonly, the light transmissive layer is formed of an ultraviolet curing resin, such as an acrylic polymer material or the like.
However, if the numerical aperture of the objective lens of the optical pickup is increased, as proposed above, the working distance of the objective lens (the distance between the objective lens and the optical recording medium) must consequently be reduced. If this working distance is not reduced, and a working distance similar to that of a conventional objective lens is used, the objective lens must be very large and heavy, and as a result, the access time for reproducing information from the optical recording medium is slow and the tracking servo and focus servo are less exact. It has therefore been proposed to reduce the weight of the objective lens by bringing the objective lens closer to the optical recording medium, thereby shortening the working distance therebetween.
However, when this working distance is reduced, it is thereafter possible that the optical pickup lens will sporadically come into contact and collide with the optical recording medium. The transparent disc substrate and light transmissive layer are generally formed of plastic such as a polycarbonate, UV curable resin, or the like, because plastic is inexpensive and permits guide grooves to be easily formed therein, and also permits other pits and grooves for recording information to be easily formed therein by ejection molding. Therefore, when the optical pickup collides with the optical recording medium, it is possible that the optical recording medium will be broken. Additionally, if the objective lens of the optical pickup collides with the optical recording medium, the light transmissive layer may be easily scratched or deformed, since the plastic (such as polycarbonate, UV curable resin, or the like) which forms a transparent disc substrate or light transmissive layer is a very soft material as compared with the glass forming the objective lens. Therefore, the objective lens may crack the transparent disc substrate, or scratch the surface of the light transmissive layer. Since information is reproduced from the recording medium via the irradiation of light, any scratches in the surface thereof may disrupt the light path, and therefore reduce the effectiveness of the reproduction of the information from the optical recording medium.
Furthermore, a holder for holding the objective lens is typically formed of a plastic material. The distance between the holder and the optical recording medium is typically smaller than the distance between the objective lens and the optical recording medium, since the holder must secure the lens therein. Thus, in this configuration, the holder might collide with the optical recording medium. It is possible that the plastic of the holder for the objective lens may scratch the recording medium, if the plastic forming the objective lens holder is harder than the plastic forming the optical recording medium. However, if the holder is formed of a material that is softer than the material forming the optical recording medium, it is possible that the holder will be deformed, thereby disrupting the positioning of the objective lens.
Additionally, if any dust is positioned within the space between the holder and the recording medium, upon contact between the holder and the optical recording medium, the holder may cause this dust to scratch and/or break the transparent disc substrate or light transmissive layer of the recording medium. As noted above, since the information is retrieved from an optical recording medium via irradiation of light, any such scratches thereon may cause disc read errors. While errors in the radial direction may be small enough to be able to be corrected through an error correction process, scratches in the track direction are likely to be longer, including a longer error length, and may be more difficult to correct using error correcting techniques. Additionally, these scratches may effect the tracking or focus servo signal, thereby disabling servo control, and lowering the reliability of the reproduction of data from the optical recording medium.
Finally, any dust which may be adhered to the light transmissive layer of the optical recording medium by static electricity may make the reproduction of data therefrom difficult, may increase the error rate during a read operation, and may therefore reduce reliability of reproduction of information therefrom.
Therefore, it would be beneficial to provide an improved optical recording medium which overcomes the drawbacks of the prior art.